Method of making optical pellicles



Oct. 9, 1951 P. 1 PRYOR 2,570,273

` METHOD oF MAKING OPTICAL PELLICLES Filed sept. 2o, 1948 I J lV Y K oooaoooooaooooooooa /4 /9 47 /z INVENTOR. PJUA A PF 0E BY ML/ Patented Oct. 9, T951 UNITED STATES PATENT OFFICE `(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 5 Claims.

The invention described herein may be manufactured and used by or for the GovernmentJ for governmental purposes without payment to me of any royalty thereon.

'I'his invention relates to optical pellicles and the method of making them but more particularly to the method of stretching standard manufactured sheet plastic material and framing the stretched portion for use as optical membranes.

Optical pellicles are usually defined as thin optical membranes held in frames in a stretched or taut condition. Because of the thinness of the membranes being about one thousandth to five thousandths of `an inch, the optical effect of variations in the planeness ofthe two surfaces is very slight which is quite an advantage over glass plates where wedge angles thereof aifect the transmitted or reflected light. Such pellicles are often used, when coated with a semireiiecting surface, as transmitting mirrors or beam splitters. When dyed with a transparent color, they are used as optical filters. Y

The usual method of making pellicles is to iiow lacquer or plastic resin plasticizer solution onto a polished plate such as glass and, after a reasonable degree of hardness is reached, the thin lm is stripped olf and mounted on a rigid frame, usually done under water. The pellicle and frame arethen allowed to cure further, the plasticizer, solvent and moisture subsequently removed, and the membrane further tightened until a reasonably fiat pellicle results. Pellicles made in this manner require considerable time in processing the membrane which processing operations also require very delicate and careful handling of the membrane. Curing of the membrane in its frame causes the membrane to become very tight which oftentimes results in breakage from shock or tension. The delicate manner in which the mem- `brane must be handled, the time required for propolyvinyl butyrate, polystyrene, methyl methac- .Y opening I3.

rylate, allyl ester monomer and coated gelatine. Sheets of this material are stretched or held taut while at ordinary temperature and free from solvents or softeners to produce .a drum-head like lm and the stretched portion secured in a frame. The frames may be of any shape as dictated by the use to which the pellicles are to be put but are ordinarily circular.

It is a primary object of this invention to provide optical pellicles of sheet plastic.

It is another principal object of this invention to provide a method of producing optical pellicles from sheet plastic.

It is a further object of this invention to provide optical pellicles capable of withstanding shock, temperature and humidity changes by resiliently mounting an optical membrane in a frame. V

It is still another object of this invention to provide a method of tensioning an area of thin sheet plastic material and securing a portion of the tensioned area in a retaining frame for producing optical pellicles.

These and other objects and advantages will become more apparent by reference to the speci-r cation when taken in conjunction with the accompanying drawing forming a .part thereof, in which;

Fig. 1 shows one form of tensioning and framing plastic sheet material in accordance with this invention;

Fig. 2 shows a modification of the form of tensioning and framing sheet plastic material;

Fig. 3 shows another modiiication of the method of tensioning and framing sheet plastic material; and

Fig. 4 shows still another modication of the tensioning and framing means.

Referring to the several views of the drawing in which like characters of reference refer to corresponding parts, and more particularly referring to Fig. 1, there is shown a piece of sheet plastic material IU that is laid across an element II having a flat top surface I2 and a central While the peripheral shape of the flat top surface is not important, it is preferable that the element I I be circular and iiXed stationary in any well known manner. A tensioning member I4, of substantially .the same shape and diameter as the top surface I2 of the element I I lingers I5 engage the plastic sheet material I0 on the side opposite the stationary element I I and force the plastic sheet outwardly in all directions, the fingers being arranged to offer greater resistance to slip with the plastic sheet than the stationary element I I. The degree of tautness of the plastic sheet is determined by the downward force applied to the tensioning member I4. While the plastic sheet I0 is being held in this taut condition, a pair of frame members I6 and I1 having corresponding openings I8 and I9 are placed on opposite sides of the stretched sheet I0 with the drilled openings I8 and I9 in alignment, the plastic sheet is punched at the places where the drilled openings occur and the two frame members I6 and I1 fastened together with rivets 20 clamping the plastic sheet therebetween. Plastic sheet I Il is then released by the tensioning member I4 and the frame cut out of the sheet at its outer perimeter. The resulting pellicle comprising of the frame stretched plastic sheet may then be polished, coated, or otherwise processed for the use to which it is to be put.

Protection against abrasion and changes of humidity can also be achieved by the selection of the proper coating. This coating can be applied when the sheet is manufactured, or it can be applied during the process of forming the pellicles, or when the pellicles are finished. It can be applied in any of the known ways such as dipping, brushing, flame-spraying, evaporation, or sputtering.

In order to eliminate minor optical imperfections which occur on the surface of some plastic materials, a polishing step may be introduced in this method. This polishing step may consist of buiiing, or flaming, or it may be accomplished by pressing the material between two polished heated plates. For some purposes, some materials will not reduire polishing. The surface of the material such as regenerated cellulose or gelatine can be polished in the above manner if the proper coating is selected.

Referring now to Fig. 2, there is shown another modication of the means for stretching the sheet plastic and framing means. In this modification, plastic sheet II] is held in a substantially flat position by annular clamping iaws 25 and 26. An annular tensioning member 2'! having a rounded top rim portion 28 and of a diameter to slidably rit within the lower jaw 21 is movable to engage the underside of the plastic sheet to stretch it and hold it in the stretched condition while frame members 29 and 3f! are fixed to opposite sides of the plastic sheet. As illustrated in this figure, the frames 29 and 30 are cemented or held by an adhesive substance to the plastic sheet IQ. When the cement or adhesive is fast, the pellicle so formed is cut from the plastic sheet, Il) for use.

In the modification of Fig. 3, an annular stationary member 35 has the plastic sheet I!) laid over a flat top portion 3B thereof in a manner similar to that shown and described for Fig. 1. A tensioning member 31 has an annular depending portion 38 adapted to slidably fit over the annular stationary member 35. A resilient ring of rubber or the like 39 having a depending lip portion 4I! is retained within the annular depending portion 38. The depending lip portion 4t of the resilient ring 39 is anged outwardly such that downward movement of the tensioning member 31 brings the depending lip 4Ilinto engagement with the plastic sheet and a compressive force on the resilient ring forces the lip 4U radially outward pulling the plastic sheet radially outward making it uniformly taut. Two annular ring parts 4I and 42 are then clamped on the plastic sheet Ill by any suitable clamping means and the plastic sheet cut at the outer periphery. A metal band 43 (shown in phantom lines since this band is only positioned after the pellicle is cut from the plastic sheet) is then placed over the outer periphery of the frame parts 4I and 42 and spun or crimped over the edges thereof to hold the plastic sheet in place forming a pellicle for use.

Fig. 4 shows the plastic sheet Iii being stretched by placing the sheet over a hoop 45 and slipping a second hoop 46 downwardly over the first hoop 45 in the same manner that crochet hoops are used to tighten a portion of cloth. While the plastic sheet is held taut, a composite frame of two rings 41 and 48 of resilient material and two stiff rings 49 and 5U are cemented or otherwise secured in alignment on opposite sides of the sheet IIJ with the resilient rings being next adjacent the opposite sides of the plastic sheet and between the stiff rings. The pellicle so formed is then cut out of the plastic sheet for use. Any local stresses or increased stretch in the plastic sheet otherwise endangering rupture of the plastic membrane is relieved by the rings 41 and 43 due to their resilience.

While, for the purpose of illustration, each o1 the iigures show modiiications of methods of tensioning the plastic sheet as well as methods of framing the plastic membrane, it is to be understood that the framing methods can be used in any combination with the methods of tensioning the plastic sheet. All pellicles so produced may be processed for various uses as formerly described of the pellicle in Fig. 1.

It is to be further understood that other modifications and changes may be made in the details of construction and the method steps without departing from the spirit and scope of this invention and I desire to be limited only by the scope of the appended claims.

I claim:

1. A method of producing optical pellicles comprising, stretching a thin plastic sheet at ordinary temperatures and while free from solvents and softeners to obtain equal stress in all directions, securing a stiff frame to a portion of said stretchedv plastic sheet and cutting said framed portion 0i" said plastic sheet from the whole plastic 2. A method of producing optical pellicles com prising, stretching a thin sheet of plastic material to obtain equal stress in all directions, placing stiff rings in alignmentI on opposite faces of the stretched plastic sheet and securing these rings together with said plastic sheet secured therebetween, cutting said plastic sheet at the outer perimeter of said rings, polishing the plastic sheet membrane formed in said frame to obtain optical flatness thereof, and coating said membrane for the purpose it is to be used whereby the framed plastic sheet membrane provides an optical pellicle of good optical qualities.

3. A method of producing optical pellicles comprising, stretching a thin sheet of transparent plastic material, adhesively securing similar stiff rings in alignment to opposite surfaces of the stretched portion of said plastic sheet, cutting the optical pellicle so formed from said plastic sheet at the outer perimeter of said stiff rings, and polishing and coating the framed plastic sheet for optical use.

4. A method of producing optical pellicles comprising; stretching a thin sheet of .transparent plastic to obtain equal stress in all directions;

securing a ring of resilient material to opposite surfaces oi' said plastic sheet, said rings being of equal diameter and in alignment; securing an inflexible ring equal in diameter to said resilient rings to the surface of each resilient ring and'in 5 alignment therewith to produce a framed membrane optical pellicle; cutting the pellicle, .l so formed from said plastic sheet; polishingV said pellicle membrane to produce optical flatness and coating said pellicle membrane for the optical use to which the pellicle is to be put.

5. The method of making a pellicle from plastic membrane which comprises stretching said membrane outwardly within an elastic ring. while the membrane is at ordinary temperature and free from solvents and softeners enclosing a central portion of the so-stretched membrane by a stiff ring and flattening the membrane enclosed by the stiff ring to Optical parallelism by smoothing the membrane between polished ilat hard surfaces.

PAUL L. PRYOR.

f 6 REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number lName Date 1,333,744 y Wente Mar. 16. 1920 1,404,668 Tillyer Jaxf. 24, 1922 1,582,564 Vredenburg Apr. 27, 1926 1,926,649 Reinhold Sept. 12, 1933 1,945,933 Chilowsky et al Feb. 6, 1934 2,058,703 Malivert Oct. 27, 1936 2,174,269 Land Sept. 26, 1939 2,182,585 Green Dec. 5, 1939 2,397,242 Chubb Mar. 26, 1946 2,401,044 Brandt May 28, 1946 OTHER REFERENCES plexiglass-Fabricating Manual-Rohm and Haas Co.-Washington Square, Philadelphia, Pa.-9th Edition--1945--Pages 17-21-Copy in 88-5512. 

